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Posted By Topic: Caravans with hot-water cylinders

ShaneR
Jan 02 2018 07:45

Hi all
I came across a 70's caravan with a hot-water cylinder that kept tripping the pillar supply RCD. (Caravan park)


Caravan had a current WoEF and had been converted for RCD Supply.

Heating elements leak, what are the options for successful operation of this caravan?

Hot water cylinder had its own circuit breaker so just disabled.
   

ShaneR
Jan 02 2018 07:48

I think i read somewhere about special RCD compatible elements?

If so how common are they?
   

AlecK
Jan 02 2018 08:21

An IR test on the w/h will provide the answer.

Not all elements leak, and of those that do some leak more than others. Elements of modern manufacture are less prone to absorbing atmospheric moisture, due to better seals being used at ends.
But with w/h elements the problem is usually corrosion of the sheath.

Similarly not all RCDs rated "30 mA" trip at 30 mA, which explains why only one RCD tripped.

If you have left the element N connected, the problem will re-occur as soon as you apply some other similar load , because there is still an alternative path for return current.

   

ShaneR
Jan 02 2018 17:35



"Similarly not all RCDs rated "30 mA" trip at 30 mA, which explains why only one RCD tripped."


When I say RCD converted as was explained to me the inspector just removed the link and applied WoEF and said problem solved.........But that's another story

"If you have left the element N connected, the problem will re-occur as soon as you apply some other similar load , because there is still an alternative path for return current."


Yes, I wish I could download the data from you just like in the matrix. :)


Thanks again AlecK
   

ShaneR
Jan 02 2018 18:58

I have always assumed that any value below 1 Megohm will trip and RCD.

We know we can go as long as 0.01 Megohm for heating elements.

Got my calculator out and have concluded

1 Megohm = will leak 0.023 millamps
0.01 Megohn = will leak 23 millamps

???

What level should a caravan fail and EWoF?

I assume you disconnect the heating element then test above 1 megohm.

Then test the heating element separately


0.05 Megohn = will leak 4.6 millamps

You wouldn't really want to go below this figure?

Assuming i've done the maths right?




   

Sarmajor
Jan 02 2018 20:30

The rules for a caravan are no different than for any installation with regard to RCD tripping current limits. The rules for equipment containing heating elements also apply in this instance.
A bit of further testing should reveal where the problem is.
If it is the element causing the tripping due to leakage the options are to run the van from a non RCD power supply to get the element hot enough to drive out any moisture. If this works then regular use of the caravan will prevent this from becoming a problem in the future.
If the element is sufficiently lot in IR then replacement will be the best option.
Given that the RCD in the van did not operate further investigation is required and you will probably need a clamp ammeter that can read ma to measure the leakage of individual items.
   

AlecK
Jan 03 2018 10:36

"When I say RCD converted as was explained to me the inspector just removed the link and applied WoEF and said problem solved.........But that's another story"

So, yet another "inspector" who shouldn't be one. Unfortunately not an uncommon story.

Removal of N-E link is PEW, and triggers two clauses in "3001". Firstly 3.10.3 requires a full set of earth tests. That's because most old-style caravans with links had the incoming PEC connected to the N-bar - juast like fixed installations of the same period. Removing the link means there is NO connection between the caravan earthing system and the incoming PEC, so no path for fault current to trip a protective device when a fault occurs. Basically a death-trap, waiting for a person to complete the circuit.
Secondly there's "upgrade" clause 3.3.5: when any additions or alterations are carried out on any caravan - specifically including the removal of N-E links - RCD protection must be fitted covering all final subcircuits.
   

AlecK
Jan 03 2018 10:37

For the RCDs, the nominal rating is when they MUST trip at or before.
Manufacturing variables mean that to ensure tripping at no more than 30 mA, the actual set point will be a bit less - and not identical for every RCD. Tripping in mid-twenties of milliamps is common - some RCD testers have a "ramp" test that gradually increases current to determine the actual trip point.

OK in this case the only RCD was in the supply pillar, but that wasn't clear in OP. It's why I use an independent supply when testing caravan RCDs, because I need to trip the one in the caravan and not the one on the fixed-wired supply, which may well have a lower tripping point and/or a slightly faster action.
   

pluto
Jan 03 2018 11:08

For your information
AS/NZS 61008 (RCCB) , AS/NZS 61009 (RCBO) both require that the mnimum level of residual current tripping is 22 mA and all RCDs (RCCB or RCBO) must have tripped by 30 mA maximum.

An isolating transformer with one side of the secondary winding connected to earth connection of the supply lead is a suitable way of the testing the on-board RCDs. Because the 230 V current required for the purpose made RCD tester operation use a 1 kVA isolating transformer and turn off all other loading in the caravan by tripping the over current protective device(s).

BTW if you DO NOT want to destroy your expensive purpose made RCD tester, don't use an inverter or a generating set (unless it is an inverter type with a clean waveform output)supply.
   

DougP
Jan 03 2018 17:22

Just wondering how you get an "independent supply" Alec?
   

AlecK
Jan 03 2018 17:49

Pluto has already given two answers: one of which is truly independent(can be used with no other supply available); and the other just provides separation between the 2 RCDs.

   

ShaneR
Jan 08 2018 18:45

Thanks guys